Seasonal population structure,vertical distribution and migration of the chaetognath Sagitta elegans in the Celtic Sea

The biology of the chaetognath Sagitta elegans Verrill has been much researched, but detailed studies of population structure have generally been conducted in coastal water where dynamic tidal conditions may cause difficulty in interpretation of data. The resolution of sampling examining vertical distribution and diurnal migration has also been rather coarse. During a series of eight cruises to a seasonally thermally stratified sampling site in the Celtic Sea in 1978 and 1979, detailed vertical zooplankton profiles were taken to study the seasonal population structure, vertical distribution and migration of this species. The overwintering stock of S. elegans (22 to 52 individuals m^-2, 0 to 90 m) had a wide range of lengths (5 to 20 mm) and matured in 1978 from early March, spawning several times before dying out by late July. Young produced by the overwintering stock started to mature in July and population numbers reached their highest in August (2483 m^-2, 132.8 mg C m^-2) when sea temperature peaked (17.1°C). By October, the population of S. elegans declined (284 m^-2), which was thought to be due to a combination of lower sea-water temperature, competition for and availability of food, and predation. Because of the length range of the overwintering population (5 to 20 mm), it is assumed that reproduction continued at a low level over the winter, although eggs were not found in January and February, the coldest months of the year. In summer, the smallest S. elegans (2 to 6 mm) were found in the near-surface waters and did not migrate, but as their lengths increased they occupied deeper depth ranges and a portion of the population started to migrate diurnally. Individuals which did not migrate and stayed in the warmer surface waters, or those which migrated into it, matured faster than those remaining in the colder water below the thermocline. Migration to surface waters by mature individuals seemed to be stopped by high surface temperatures (17°C) and a sharp thermocline (3 C°). As sea temperature increased during the year from the winter minimum of 7.7°C, S. elegans matured at a progressively shorter length (14 mm in March 1978 to 10 mm in August). There are probably only three generations of S. elegans a year in the Celtic Sea.